Housing for a circuit that is to be implanted in-vivo and process of making the same

ABSTRACT

The present invention provides a biocompatible circuit assembly that includes a circuit encased within a housing. In some embodiments, the housing is a PMMA housing and before the circuit is enclosed within the housing the circuit is encased within a brick of epoxy.

This application is a continuation of U.S. patent application Ser. No.13/171,711, filed on Jun. 29, 2011, which is a continuation of U.S.patent application Ser. No. 10/825,648, filed Apr. 16, 2004, both ofwhich are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a circuit housing, and, morespecifically, to a housing for a circuit designed to be implantedin-vivo (i.e., an implantable circuit).

2. Discussion of the Background

There are several applications that require a circuit to be protectedfrom the environment in which the circuit is intended to operate. Forexample, a human implantable glucose sensor circuit must be housedwithin a suitable housing to both protect the sensor from the human bodyand to protect the human body from the sensor. U.S. Pat. No. 6,330,464,the disclosure of which is incorporated herein by this reference,discloses such a sensor.

A housing encasing an implantable circuit should have at least some ofthe following characteristics: (1) the ability to protect the electroniccircuitry of the sensor from the ambient in-vivo chemical and physicalenvironment, (2) the ability to protect tissue adjacent to the sensorfrom any adverse reaction which could result as a consequence of contact(or leachables) from within the circuitry—in addition, beyond theadjacent tissue, the encasement must not permit leachables of anydetectable significance into the general body environment; (3) theability to permit wireless electronic communication between thecircuitry and an external reader for power and signal; (4) the abilityto permit free passage of wavelengths of light necessary for opticalfunctioning of the sensor; (5) the ability to support the surfacechemistry required to form a chemical recognition “front-end”; (6) thehousing should be high volume manufacture-able; (7) the housing must benon-toxic and “biocompatible”; and (8) provide a sufficiently highreliability to meet the specifications of a medical product.

SUMMARY OF THE INVENTION

The present invention provides a housing that meets many of the criteriaoutlined above. In one aspect, the present invention provides a circuitencased within a completely enclosed polymer housing. Preferably, thehousing is made of an organic polymer, such as PMMA. In someembodiments, the circuit is first enclosed within a glass housing whichitself is then enclosed within a second housing, such as a housing madefrom an organic polymer. In other embodiments, the circuit is firstencased within a brick of epoxy and then the epoxy brick containing thecircuit is enclosed within a housing.

In another aspect, the present invention provides a method for enclosinga circuit in a polymer housing. In one embodiment, the method mayinclude the following steps: (a) placing the circuit in a mold; (b)pouring a formulation into the mold so that the formulation completelysurrounds the circuit, wherein the formulation comprises monomers; and(c) polymerizing the monomers. In step (b), all of the formulation neednot be poured at once. For example, in some embodiments, the formulationis poured into the mold until the mold is half full and then after adelay additional formulation is poured into the mold. In someembodiments, the monomers may be MMA monomers. The formulation mayfurther comprise pre-polymerized PMMA.

In another embodiment, the method may include the following steps:inserting the circuit into a polymer housing; injecting an optical epoxyinto the polymer housing to fill the spaces between the circuit and theinside walls of the housing (in some embodiments the injection is fromthe bottom up to force out trapped air); capping an open end of thehousing; placing the housing containing the optical epoxy and thecircuit into a pressure vessel and increasing the pressure andtemperature within the vessel; allowing the optical epoxy to cure; andremoving the housing from the pressure vessel.

In another embodiment, the method may include the following steps:inserting the circuit into a glass housing; injecting an optical epoxyinto the glass housing to fill the spaces between the circuit and theinside walls of the housing; injecting an optical epoxy into a polymerhousing; inserting into the polymer housing the glass housing containingthe circuit; capping an open end of the glass housing; and capping anopen end of the polymer housing.

The above and other features and advantages of the present invention, aswell as the structure and operation of preferred embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, help illustrate various embodiments of the presentinvention and, together with the description, further serve to explainthe principles of the invention and to enable a person skilled in thepertinent art to make and use the invention. In the drawings, likereference numbers indicate identical or functionally similar elements.Additionally, the left-most digit(s) of a reference number identifiesthe drawing in which the reference number first appears.

FIG. 1 illustrates one embodiment of a circuit assembly according to thepresent invention.

FIG. 2 is a flow chart illustrating a process, according to oneembodiment, for encasing a circuit within a polymer housing.

FIG. 3 is a cross sectional view of a circuit assembly according to anembodiment of the invention.

FIG. 4 is a flow chart illustrating a process, according to anotherembodiment, for encasing a circuit within a polymer housing.

FIG. 5 is an exploded view of a circuit assembly according to anembodiment of the invention.

FIG. 6 is a cross sectional view of a circuit assembly according toanother embodiment of the invention.

FIG. 7 illustrates a circuit assembly according to another embodiment ofthe present invention.

FIG. 8 is an exploded view of a circuit assembly according to anotherembodiment of the invention.

FIG. 9 is a flow chart illustrating a process, according to anotherembodiment, for encasing a circuit within a polymer housing.

FIGS. 10A and 10B illustrate a circuit covered with different amount ofepoxy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates one embodiment of a circuit assembly 100 according tothe present invention. As shown in FIG. 1, the present inventionprovides an assemblage including a circuit 101 housed within a fullyenclosed housing 102. Preferably, as shown in FIG. 1, housing 102 iscapsule shaped, but other shapes may be used. Circuit 101 may be anelectronic circuit having a printed circuit board 110 and one or moreelectrical and optical components 112 attached to the circuit board 110.Circuit 101 may include a conventional sensor, such as the sensordescribed in U.S. Pat. No. 6,304,766. The housing 102 may be a housingmade from PMMA, which is a polymer of methyl methacrylate (MMA)monomers, or from other organic polymers.

FIG. 2 is a flow chart illustrating a process 200, according to oneembodiment, for creating circuit assembly 100. Process 200 may begin instep 202, where a polymerization initiator is added to a mold. In step204, an encasement formulation containing monomers is poured into themold (e.g., filling the mold halfway). In step 206, circuit 101 isplaced in the mold. In step 208, more of the encasement formulation ispoured into the mold so that the circuit is completely immersed in theencasement formulation. In one embodiment, the encasement formulationincludes monomers. In one embodiment, the encasement formulationconsists of or essentially consists of MMA monomers. In this manner, onecan encase circuit 101 in a polymer housing.

In some situations, for example, situations where the formulationincludes MMA monomers and circuit 101 is relatively large, circuit 101can become severely damaged during the polymerization process (i.e.,during step 206). The cause of this damage is usually attributed to theshrinkage that occurs naturally during polymerization of MMA. In thejoining of bonds between monomers contained within a neat solution ofMMA, the intermolecular spacing is reduced within a polymer as thereaction progresses. This is a well-known phenomena and typical of most,if not all, polymer reactions. The net volumetric shrinkage that occursduring the polymerization of PMMA from neat monomer solution isapproximately 14%.

This shrinkage can, in some circumstances, create a particular problemwhen using PMMA as a circuit housing because, as the encasement reactionprogresses, and the viscosity increases as the shrinkage occurssimultaneously, the electrical components 112, which are mounted on thecircuit board 110 typically with conductive epoxy, are pulled from thecircuit board 110 during the polymerization process.

The relative strength of the conductive epoxy used to hold thecomponents 112 in place, which conductive epoxy is formulated primarilyand maximally for its electrical conductance and cure properties, doesnot have sufficient mechanical strength to withstand the pull and stressfrom PMMA shrinkage as the encasement reaction progresses. Consequently,some attempts to encase a circuit from an MMA monomer encasementformulation result in a non-functional circuit because of un-repairablemechanical damage.

To solve this problem, one aspect of the present invention is a methodby which the polymerization reaction can be conducted without damage tothe encased circuit 101. Because pre-polymerized PMMA of large molecularweights (approximately up through 1 million+mw) can be dissolved in MMAmonomer, and because the shrinkage is a direct result of bonds formingfrom discrete monomers, one possible solution is to formulate theencasement formulation to include a portion of MMA monomer and a portionof pre-polymerized PMMA dissolved within the MMA monomer.

The net shrinkage is proportional to the amount of monomer which isreacted to become polymer within the overall volume. If the overallencasement formulation volume, is portioned to include, for example,about 70% pre-polymerized PMMA, and about 30% un-reacted MMA monomer(into which the 70% PMMA has been dissolved), then the degree ofshrinkage which occurs drops in direct proportion to the monomercomponent within the overall volume. In practice, an encasementformulation of 100% MMA monomer shrinks volumetrically about 14%overall. By dropping the formulation to only 30% MMA, shrinkage in theamount of approximately 0.3×14=4.3% would be expected. In practice,approximately 4% shrinkage is measured from making this improvement.

Accordingly, the result of altering percent solids provides animprovement in system stress during encasement by reducing shrinkagefrom, for example, 14% to 4% by reformulating MMA/PMMA specifically forthe encasement process. Formulation ratios of 60-80% PMMA in MMA arepreferred, although not required, because of a present practicallimitation. Although to a point, higher ratio values would be expectedto reduce shrinkage proportionately, and further reduction in shrinkagemay be possible. As a practical matter, the solution viscosity becomesextremely high at these higer ratio levels making the high solidssolution extremely difficult to handle, transfer, etc.

In some situations, however, even with 4% shrinkage, which is a greatimprovement over 14%, some percentage (about 40-50%) of circuits 101 cannot withstand the 4% shrinkage of the encasement. The surviving circuitstend to have greater amounts of conductive epoxy to increase mechanicalstrength slightly of the surface mounted parts. However, conductiveepoxy is not sufficiently strong, and to increase the amount used perconnection beyond good manufacturing standards would then create otherproblems. Another important consideration is for wire-bonded circuits.These “frog hair” gold wires are typically 25 microns in diameter whichis about ⅓ to ¼ the diameter of a typical human hair. Small amounts ofmovement relative to the fixed board components can rip these wires fromthe attachments.

Accordingly, in some applications, it is desirable to mechanicallystrengthen the circuit 110 to allow it to withstand the remainingshrinkage from the PMMA encasement cure reaction.

One way to mechanically strengthen circuit 101 to allow it to withstandthe remaining 4% shrinkage from the PMMA (70/30) encasement curereaction, is to reinforce the circuit with a pre-applied epoxy layer.For example, following assembly of the electrical components to thecircuit board and cleaning of the assembly, an epoxy is applied over thecircuit, which epoxy both under-fills and overfills the componentsattached to the circuit board. Surprisingly, it was discovered that thissolution works best when the applied epoxy covers the components in sucha way as to result in a relatively “smooth” surface topology, but thisis not a requirement. This “smooth” surface topology is illustrated inFIG. 10A. For comparison, FIG. 10B shows a “non-smooth” epoxy coating.As shown in FIG. 10A, the surface 1002 of the epoxy coating is smooth orsubstantially smooth.

Although the epoxy adequately strengthens circuit 101 against damagefrom the shrinking polymer, the resultant stress caused by the remaining4% shrinkage then becomes manifest as de-lamination between theadjoining surfaces of epoxy and PMMA within the final encasement. Asmentioned above, it was discovered that if the surface was smoothed bythe volume and application of the epoxy pre-coat, not allowing the PMMAto get a “grip” within the surface topology, then de-lamination was lesslikely to occur. The stress from the 4% remaining shrinkage is thenabsorbed as internal stress within the PMMA encasement body itself. Thisstress may be removed in a conventional way by annealing in a finaloperation.

Some or all of the epoxy used to reinforce the circuit 110 may, in someembodiments, include a light blocking pigment (such as black orwavelength specific) which prevents unwanted light propagation andscatter about the circuit, thereby increasing the optical signal tonoise ratio of the system.

In some embodiments, to prolong the life of the circuitry 101, it may bedesirable to prevent molecular water vapor that has seeped through thehousing 102 from condensing to become liquid water. If liquid watercannot form from the water vapor, then potential ion contaminantspresent cannot become solvated, which can lead to circuit failure.

One way to prevent the water vapor from condensing is to prevent theformation of heat induced bubbles in the encasement polymer. MMA monomeris extremely volatile. The polymerization reaction of MMA to PMMA isalso exothermic. The exothermic heat yield from a typical reaction begunat room temperature will commonly increase the temperature as thereaction progresses to a point where the remaining un-reacted monomerwill boil and create bubbles of all sizes trapped within the curedpolymer. To prevent any possibility of heat induced micro-bubbles andvoids within the housing where water vapor could condense, substantialoverpressure may be used during the polymerization reaction. Morespecifically, in a preferred embodiment, a mold containing PMMA/MMA isplaced within a pressure reactor that is then pressurized to a pressurethat exceeds the vapor pressure of MMA monomer at the polymerizationtemperature. This pressurization process both prevents bubbles andprovides a very close mechanical surface bond with the underlying epoxycoat which does not delaminate once formed. The housing is clear andwithout bubble or void defects to prevent water from condensing, and asan important byproduct, provides excellent optical clarity withoutbubble defect.

Referring now to FIG. 3, FIG. 3 is a cross sectional view of circuitassembly 100, according to one embodiment, along line A. As shown inFIG. 3, the circuit 101 may be fully encased within a brick of epoxy 302(or “epoxy brick 302”), which is encased within housing 102.

FIG. 4 is a flow chart illustrating a process 400, according to anotherembodiment, for creating circuit assembly 100. Process 400 may begin instep 402, where a housing 500 (e.g., a sleeve 500 or tube or otherhousing having an open end) (see FIG. 5) is created along with a plug504 for plugging the opening in the housing. For example, a cylindricalsleeve 500 and plug 504 may be machined from a polymer rod, such as arod of PMMA or other organic polymer. As shown in FIG. 5, sleeve 500 mayhave a notch 592 adjacent to the open end 594 of sleeve 500. If sleeve500 and plug 504 are made from PMMA, the PMMA sleeve and plug may beannealed at approximately 80° C. for about four hours (step 403).

In step 404, epoxy is applied over the circuit 101 so that the circuitis partially or fully encased within an epoxy brick 502, thereby formingan assembly 503. In step 406, assembly 503, sleeve 500 and plug 504 arecleaned. For example, assembly 503, sleeve 500 and plug 504 may becleaned by rubbing a Q-tip with IPA on the surfaces thereof. In step408, an optical epoxy is prepared. EPO-TEK 301-2 Epoxy from EpoxyTechnology of Billerica, Mass. and other epoxies may be used as theoptical epoxy.

In step 410, the circuit encased within the epoxy brick (i.e., assembly503) is placed into the sleeve 500. In step 412, the prepared opticalepoxy is injected (i.e., introduced) into sleeve 500. Preferably, nobubbles in the optical epoxy are formed during step 412. In step 414,the plug 504 is placed into the open end of sleeve 500, thereby sealingthe open end of the sleeve.

FIG. 6 is a cross sectional view, according to one embodiment, of thecircuit assembly 100 along line A after step 414 is performed. In theembodiment shown in FIG. 6, the circuit 101 is fully encased within anepoxy brick 502. The epoxy brick 502, which houses circuit 101 is placedwithin sleeve 500, which may be a cylindrical sleeve. When sleeve 500 isa cylindrical sleeve and when circuit 101 is fully encased within theepoxy brick, it is preferable that the distance between the upper righthand corner and lower left corner of epoxy brick 502 be equal to orslightly less than the inner diameter of sleeve 502. That is, in someembodiments it is preferable that w=sqrt((d*d)−(h*h)), where w is thewidth of assembly 503, h is the height of assembly 503, and d is theinner diameter of sleeve 500. In embodiments where the assembly 503 doesnot have a uniform width or has a circular shaped cross section, thenthe maximum width of the assembly may be equal to or slightly less thanthe inner diameter. As illustrated in FIG. 6, the optical epoxy (e.g., arefractive index (RI) matching epoxy) fills spaces between assembly 503and sleeve 500.

Referring back to FIG. 4, in step 416, the new assembly (i.e., thesealed sleeve containing the epoxy and assembly 503) is placed into apressure vessel. In step 418, the pressure within the vessel isincreased to about 125 psi using Nitrogen or other inert gas. In step420, the optical epoxy is cured for an amount of time (e.g., 20 hours)at a predetermined temperature (e.g., 40° C.). After the predeterminedamount of time has elapsed, the assembly is removed from the pressurevessel and then final machined (step 422).

The method described above allows the possibility of annealing a PMMAhousing before encasement without putting any additional stress on thecircuit 101.

FIG. 7 illustrates an alternative circuit assembly 700 of the presentinvention. Circuit assembly 700 is similar to circuit assembly 100 inthat assembly 700 includes a circuit 101 housed within a housing 102.However, in assembly 700, the circuit 101 is also housed within a glasshousing 702 (e.g., a tube or other shaped housing), which itself ishoused within the housing 102. The glass housing 702, in someembodiments, is closed at one end and open at the opposite end. The openend may be plugged by a glass ball 704 or other suitable plug. FIG. 8 isan exploded view showing the components of assembly 700, according toone embodiment. Glass housing 702, in some embodiments, may beconstructed from an infra-red (IR) blocking glass.

FIG. 9 is a flow chart illustrating a process 900, according to oneembodiment, for making assembly 700. Process 900 may begin in step 902,where a sleeve and a plug, such as sleeve 500 and plug 504, are created.

In step 904, the sleeve and plug are annealed. The sleeve and plug maybe annealed at 80° C. for about four hours. In step 906, the components(e.g., sleeve 500, plug 504, glass housing 702, glass ball 704, epoxybrick 502, etc.) are cleaned. For example, sleeve 500 and plug 504 maybe cleaned in an ultrasonic bath with IPA followed by a rinse step, andglass housing 702 and glass ball 704 may also be cleaned ultrasonicallywith KOH/alcohol solutions and then rinsed with water.

In step 908, a bonding agent is applied to the glass housing 702 andglass ball 704. The bonding agent used may be trimethoxy[2-(7-oxabicyclo [4.1.0]hept-3-yl)ethyl] silane, which may be purchasedfrom Sigma-Aldrich Corporation (catalog no. 413321)

In step 910, a batch of optical epoxy is prepared. In step 912, theepoxy coated circuit board is inserted into the glass housing. In step914, some of the prepared epoxy is injected into the glass housing 702.

In step 916, some of the prepared epoxy is injected into the sleeve 500.In step 918, glass housing 702, which houses the circuit, is insertedinto an open end of the sleeve. In step 920, the glass ball 704 isinserted into the open end of glass housing 702, thereby sealing theopen end of the glass housing. In step 922, the plug 504 is used to sealthe open end of the sleeve.

In step 924, the sealed sleeve, which houses glass housing 702, whichhouses the circuit 101, is placed into a pressure vessel where thepressure is increased to about 125 psi using an inert gas and thetemperature is increased to about 40° C. After about 20 hours, thepressure is gradually reduced and the assembly is removed from thepressure vessel and then final machined (step 926).

Although the above described processes are illustrated as a sequence ofsteps, it should be understood by one skilled in the art that at leastsome of the steps need not be performed in the order shown, and,furthermore, some steps may be omitted and additional steps added.

While various embodiments/variations of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. Thus, the breadth and scopeof the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

What is claimed is:
 1. A method for fully encasing a circuit within a polymer housing, comprising: placing the circuit in a mold; injecting a formulation comprising monomers and polymers into the mold; and polymerizing the formulation.
 2. The method of claim 1, further comprising the step of at least partially covering the circuit with an epoxy prior to placing the circuit in the mold, wherein a sufficient amount of epoxy is used to cover the circuit so that the resulting surface topology is substantially smooth.
 3. The method of claim 2, wherein some or all of the epoxy comprises a light blocking pigment.
 4. The method of claim 3, wherein the light blocking pigment is black.
 5. The method of claim 1, wherein the polymerization step is performed in a pressure vessel where the pressure is increased to at least about 125 psi using inert gas.
 6. The method of claim 1, wherein the monomers are MMA monomers.
 7. The method of claim 1, wherein the polymers are pre-polymerized PMMA.
 8. The method of claim 7, wherein the formulation comprises between 60% and 80% pre-polymerized PMMA by volume.
 9. The method of claim 1, wherein the monomers are MMA monomers, and the polymers are pre-polymerized PMMA.
 10. The method of claim 1, wherein the formulation comprises between 60% and 80% pre-polymerized PMMA by volume.
 11. The method of claim 10, wherein the formulation comprises 70% pre-polymerized PMMA by volume and 30% un-reacted MMA monomer into which the 70% pre-polymerized PMMA has been dissolved.
 12. The method of claim 1, wherein the formulation is injected into the mold so that the formulation surrounds the circuit.
 13. The method of claim 12, wherein the formulation completely surrounds the circuit. 